1. Field of the Invention
The present invention relates to a toner for non-magnetic single component development for use in the development of electrophotography.
This application is based on Japanese Patent Application No. Hei 10-271251, the content of which is incorporated herein by reference.
2. Description of Related Art
Powder toners used in electrophotography are required to have suitable levels of electric properties such as triboelectrification and electric resistance that relate to development and transfer performance, thermal properties that relate to fixing performance and heat resistance performance (storage stability), and properties of powder such as flowability and hardness depending on the conditions of use.
Resin materials conventionally used for powder toners include polystyrenes, styrene/acrylic acid ester copolymers, styrene/butadiene copolymers, polyesters, epoxy resins, butyral resins, xylene resins, coumarone-indene resins, etc. and various proposals have been made on detailed designs of resins depending on their application.
In particular, for resins for use in heat roll fixing, improvements in the performance of fixing on transfer paper and offset resistance have been required. The fixing performance of a toner is achieved by heat melting it using a fixing roller or the like and fixing it on transfer paper and the offset performance of a toner means that the toner molten on a heating roller does not cause cold offset and causes no hot offset when it loses viscosity.
To achieve this object, many design examples have been proposed. In particular, to maintain viscoelasticity upon heat melting or prevent change in viscosity relative to temperature change, a technology has been studied which involves expansion of molecular weight distribution, imparting a crosslinking structure, application of a rubber elastic material, etc. For example, Japanese Patent Application First Publication No. Hei 1-267661 discloses a technology using these means.
For electrophotography, various methods are described in U.S. Pat. No. 2,297,691, Japanese Examined Published Application No. Sho 42-23910 and Japanese Examined Published Application No. Sho 43-24748. Generally, an electrostatic charge latent image is formed by various methods using a photoconductive substance and the latent image is developed with a developer (static charge developing toner) to obtain a visible image, which is fixed by pressurization, heating or with vapor of a solvent after it is transferred on paper, if desired, to thereby obtain a fixed image.
Many methods are known as developing methods in electrophotography. They are roughly divided into two-component developing methods using as a developer a mixture of a carrier consisting of fine particles of iron powder, ferrite, nickel, glass or the like (20-500 .mu.m) and toner and single component developing methods using a developer consisting of a toner alone. In either method, generally charges are injected into the toner by triboelectrification.
Typical examples of the two-component developing method include a cascade method described in U.S. Pat. No. 2,618,552 and a magnetic brush method as described in U.S. Pat. No. 2,874,063. These method can give good images stably. However, they tend to suffer from contamination of the surface of the carrier with toner and deterioration of image quality due to a change in triboelectrification attributable to the fluctuation in the mixing ratio of the carrier and the toner and various efforts have to be made with regard to apparatus and materials used as countermeasures to prevent such.
The single component developing method, which is contemplated to obviate these problems associated with the two-component developing method, includes, for example, a method for developing using an electrically insulating magnetic toner as described in U.S. Pat. No. 4,336,318. In the method, triboelectrification between toner particles and the toner carrier or the toner thinning member, or triboelectrification between the toner particles themselves results in injecting charges into the toner so that the toner adheres to the static charge image on the photoconductor.
This developing method has the advantages that it can obviate the above-described problems of the two-component developing method and the developing apparatus can be down sized since it uses no carrier and a device which controls the mixing ratio of a carrier and a toner is no longer necessary.
On the other hand, the above method involves the formation of a magnetic brushed layer of toner on a metallic sleeve so that it is necessary for the toner to have appropriate magnetic properties, resulting in the toner containing a magnetic material such as magnetite and ferrite as an essential material in the components constituting the toner. The necessary content of the magnetic material may vary more or less depending on the conditions of development, the kind of materials, etc., but generally it can be said to be 30 to 60% by weight.
However, generally speaking, to contain a large amount of such a magnetic material as described above that has a low electric resistance and readily absorbs moisture causes a decrease in electric resistance and a decrease in moisture resistance of the toner itself and as a result it is difficult to obtain stable developing performance against the change in environment to cause a considerable fluctuation in image density or background contamination level in various environments of use.
If the proportion of the resin material contained in the toner as a binder is smaller than that in the two-component toner, it may be disadvantageous in design from the viewpoint of fixing performance. Further, in view of the use of color images which are increasingly being used recently, there are problems such that most of the magnetic material must be colored so that the colors available are limited or it is difficult to obtain sharp color image quality.
To solve the above problems of the single component developing method using a magnetic toner, there has been proposed a non-magnetic single component developing method in which the toner does not have to have magnetic properties. To achieve such a method, various apparatuses have been studied, in most of which a toner is adhered on a developing sleeve and transported to a latent image surface by virtue of electrostatic power to effect development, thus markedly differing from the conventional magnetic single component developing method in that no magnetic material is necessary as an essential component in the composition of the toner, so that it is expected that the above-described various problems originating from the contained magnetic material can be obviated.
In the non-magnetic single component developing method, use is made of a powder toner for electrophotography that contains a binder resin, a colorant, and a charge control agent as essential components.
As the binder resin, a polyester resin is used since it is necessary to secure stability in electrification and durability in continuous printing.
However, demands in the market for excellent low temperature fixing properties and offset resistance as well as durability in continuous printing have been growing higher but it has been difficult to obtain a non-magnetic single component toner which meets these properties sufficiently.